Public safety agencies around the world typically use relatively narrow band, LMR technologies such as P25 to communicate voice information and some data traffic. Characteristics of this technology are long range and high quality voice. Today, Public Safety is considering the use of new technologies, such as LTE, capable of high rate data enabling mobile data applications. This form of technology is relatively short range compared to relatively narrow band technologies. In a move to capture the benefit of both technologies, methods of integration are being considered.
Previous attempts to enable LMR across a cellular network generally select between an LMR voice call or a Push To Talk over Cellular call operating over an IP network. In other words, the protocols for each path are different.
One approach that has been used in the past is a gateway between LMR and cellular. Methods of this type are subject to a number of drawbacks including; a mismatch of audio quality on either side, an incompatible feature set on either side, and loss of ability to establish end-to-end encryption. These mismatches generally occur because the protocols used on either side of the gateway are different.
Another approach that has been considered in the past is to tunnel LMR within IP. This approach avoids the mismatch of protocols causing issues when operating across a gateway. This approach is limited to the specific core network implementation of a particular manufacturer.
Typically, LMR systems are deployed over wide areas where public safety operation is expected including both populated and unpopulated areas. Typically, cellular systems are deployed over populated areas or areas where revenue can be generated through user traffic. There are areas where LMR exists but no cellular. There are areas where cellular exists but no LMR. There are areas where both LMR and cellular exist. Historically there have been innovations that seek to communicate calls using PTToC over cellular or LMR over RF depending upon which bearer is available.
Typically LMR systems exist in two forms. The first is referred to as “conventional”. In this configuration a radio channel is used by a group of terminal devices where the communication is conducted by a repeater that is typically located on a physically high site. The user of the terminal will typically make a manual choice of channel number through choosing that channel via a selection knob on the terminal. Upon pressing a push to talk button on that terminal, the voice is then received by any other terminal that is configured to listen to that channel. In its simplest form, any terminal listening to the channel will unmute. This is one way of creating group communications.
Still considering conventional communications, a group can also be created according to a group identity. The group identity is normally a specific number that identifies the group. This identity is sent along with the voice information and now any terminal that is both listening to the channel and a member of this group will unmute.
The other form of system typically used in LMR is “trunked”. In this case, there is typically one control channel and several traffic channels. In effect, a trunked system is a centrally controlled network or trunked controller. All terminals not in a voice call will listen to the control channel. If a call is initiated then they are sent to a traffic channel. Any trunked terminal wishing to access the network must first register with the network. This represents a form of identification to ensure the terminal is permitted on the network. Registration normally takes place when the terminal is switched on, changed into trunked mode or when the terminal enters coverage of the control channel while in a trunked mode. All trunked terminals must register.
Following registration, all trunked terminals must then identify which groups they are a part of. This is required so that a trunked system can make sure there is a channel available at each site that has a terminal affiliated to the group calls target. This group information (referred to as group affiliation) is knowledge held by a central controller.
Ideally, solutions are required that integrate LMR RF channels and IP capable bearers in such a way that a user is unaware of the communication bearer being used. In other words, the terminal always appears to function as a normal LMR radio whether conventional or trunked.
An LMR system is typically Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA). A channel can be defined as a frequency, frequency pair, or time slot on a frequency or frequency pair.